This application is related to the field of magneto ignition systems. In particular, this application relates to a magneto ignition system provided with biasing means responsive to a magnetic field to enhance the operation of switching devices.
Magneto ignition systems are well known. Such systems operate by inducing a current in the primary winding of an ignition coil, allowing flux to build up in the ignition coil, and suddenly interrupting the current in primary winding of the ignition coil, causing the flux to collapse, the collapsing flux inducing a high ignition impulse voltage in the secondary winding of the ignition coil. The current in the primary winding may be interrupted by conventional ignition breaker points. Or, it is also known to interrupt the current in the primary winding in response to voltage induced in a separate trigger coil, also responsive to the rotating magnet, but having a separate magnetic path and physical displacement from the ignition coil.
It has been found desirable to use a solid state switch device which may be a transistor, or any other solid state switch device in parallel with the primary winding. It has also been found desirable to provide shunting means responsive to a trigger coil to actuate the solid state switch device towards a nonconductive state, with the shortest possible switching time, and which latches the switch device in the nonconductive state for the remainder of an ignition cycle, to reduce the inherent oscillatory behavior of such ignition systems. Without such a latching device, current may start to flow again in the ignition coil windings, resulting in oscillations that may cause interrupted energy transfer to the secondary winding, preventing optimal impulse voltage generation is said winding.
The shunting device chosen for such systems is typically a semiconductor controlled rectifier or SCR. Such a SCR may be connected to shunt or bypass the drive signal to the semiconductor switch device from a drive winding, to switch it towards a fully nonconductive state. As is known, the required operating bias and sensitivity of a SCR varies with temperature, and the sensitivity of a solid state switch device varies with temperature. A SCR requires a higher operating bias to function dependably at low temperatures, and a transistor has less resistance between its base and emitter at elevated temperatures. This combination of characteristics in such a system may result in an engine which is difficult to start at low temperature due to marginally insufficient operating bias on the SCR, and an engine which misfires or is hard to start at higher temperatures, the lowered resistance of the transistor operating to shunt the bias voltage for the SCR, and reduce it to an unacceptable level, as well as increasing current flow into the transistor, preventing it from reaching a fully-nonconductive state.
Previously, additional components were provided to insure adequate operating bias for the shunt device. Such efforts have lead to complex and expensive circuits, and have interferred with optimum operating characteristics of such an ignition system at normal intermediate temperatures.
The instant invention provides proper operation for a magneto ignition system, with a minimum of parts or components, and without interferring with the optimum operation of such a system.